1. Field of the Invention
The invention relates to a lithographic projection apparatus, and more particularly to a lithographic projection apparatus.
2. Description of Related Art
An apparatus of this type can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, the mask (reticle) may contain a circuit pattern corresponding to an individual layer of the IC, and this pattern can then be imaged onto a target area (die) on a substrate (silicon wafer) which has been coated with a layer of photosensitive material (resist). In general, a single wafer will contain a whole network of adjacent dies which are successively irradiated through the reticle, one at a time. In one type of lithographic projection apparatus, each die is irradiated by exposing the entire reticle pattern onto the die in one go; such an apparatus is commonly referred to as a waferstepper. In an alternative apparatus--which is commonly referred to as a step-and-scan apparatus--each die is irradiated by progressively scanning the reticle pattern under the projection beam in a given reference direction (the "scanning" direction) while synchronously scanning the wafer table parallel or anti-parallel to this direction; since the projection system will have a magnification factor M (generally &lt;1), the speed v at which the wafer table is scanned will be a factor M times that at which the reticle table is scanned. More information with regard to lithographic devices as here described can be gleaned from International Patent Application WO 97/33205.
Up to very recently, apparatus of this type contained a single mask table and a single substrate table. However, machines are now becoming available in which there are at least two independently movable substrate tables; see, for example, the mult-stage apparatus described in International Patent Applications WO 98/28665 (P-0071) and WO 98/40791 (P-0101). The basic operating principle behind such multi-stage apparatus is that, while a first substrate table is underneath the projection system so as to allow exposure of a first substrate located on that table, a second substrate table can run to a loading position, discharge an exposed substrate, pick up a new substrate, perform some initial alignment measurements on the new substrate, and then stand by to transfer this new substrate to the exposure position underneath the projection system as soon as exposure of the first substrate is completed, whence the cycle repeats itself; in this manner, it is possible to achieve a substantially increased machine throughput, which in turn improves the cost of ownership of the machine.
In order to achieve good image definition and layer overlay in each die, the irradiated surface of the wafer should be kept as flat and as stationary as possible during exposure of the wafer. Known lithographic apparatus addresses these demands using the substrate holder hereabove specified, on which a wafer can be placed so that its backside is in contact with the protrusions, all of which lie in a well-defined plane. By connecting the aperture(s) in the plate to vacuum generating means, the backside of the wafer can be sucked securely against the protrusions, whereby the wall serves to contain the partial vacuum thereby required; to this end, the wall geometry must be matched to a given wafer diameter, so that the wafer overshoots the wall (typically by a margin of the order of about 2 mm). The use of protrusions in this manner ensures that only a fraction of the area of the backside is actually pressed against a solid surface; in this way, the distorting effect of any particulate contamination on the backside of the wafer is minimized, since such contamination will most probably be situated in the empty spaces between protrusions rather than being pressed against the top surface of a protrusion.
A problem with this known approach is that, since the wafer rests on a discrete number of protrusions rather than a continuous flat surface, and since the backside of the surface is sucked forcefully against such protrusions, the (elastic) wafer can tend to "sag" in areas where it is not supported by a protrusion. This effect can be a particular nuisance along the edge of the wafer, where the resulting distortion of wafer flatness can result in poor-quality edge dies.